SCaNME: Location tracking system in large-scale campus Wi-Fi environment using unlabeled mobility map

https://doi.org/10.1016/j.eswa.2013.10.047Get rights and content

Highlights

  • A location tracking system SCaNME without any cumbersome work in fingerprint recording is proposed.

  • The mobility map is constructed from Wi-Fi data which have no explicit information of coordinates.

  • The performance of SCaNME has been carefully examined based on the actual Wi-Fi data in HKUST campus.

Abstract

In this paper, we propose a novel location tracking system called SCaNME (Shotgun Clustering-aided Navigation in Mobile Environment) which iteratively sequences the clusters of sporadically recorded received signal strength (RSS) measurements and adaptively construct a mobility map of the environment for location tracking. In the SCaNME system, the location tracking problem is solved by first matching the people’s locations to the location points (LPs) with small Kullback–Leibler (KL) divergence. Then, Allen’s logics are applied to reveal the person’s activities, assist the on-line location tracking and finally obtain a refined path estimate. The experimental results conducted on the large-scale HKUST campus demonstrate that the SCaNME tracking system provides better precision and reliability than the conventional location tracking systems. Furthermore, the experiments of SCaNME tracking system show its capability of providing people’s real-time locations without fingerprint calibration in large-scale Wi-Fi environment.

Introduction

Location tracking in wireless network has attracted significant interest in recent years. Knowledge of people’s real-time locations and related routine activities make many promising applications for industrial and public uses (Horng et al., 2011, Zhang et al., 2013) for childcare, elderly health care, emergency rescue, as well as management in warehouse and modern buildings (Cura, 2010, Dey et al., 2011). Moreover, by the year 2014, the GPS technology could yield more than 2.5 billion dollars device market and over 12.7 billion dollars are expected to be reported by location-based service (LBS) market at that time (Ashbrook & Starner, 2003).

GPS and the Cellular network are two established infrastructures for location tracking, but they cannot work well in some urban or indoor environments due to the poor quality of the received signal from satellites or base stations. Serious attenuation and multipath interference exists in challenging radio propagation environments, such as buildings, tunnels, and subway stations (Anisetti et al., 2011, Wong et al., 2009). Therefore, a large amount of effort has been made to track people’s locations by alternative technologies, including the radio frequency identification (RFID) (Silva & Goncalves, 2009), ultrasonic wave (UW) Yoshiga et al., 2012, ZigBee (Goncalo & Helena, 2009), Bluetooth (Mair & Mahmoud, 2012), ultra-wideband (UWB) Steiner & Wittneben, 2010, and infrared ray (IR) Chen et al., 2010. However, these technologies require additional costs of infrastructure and maintenance, great effort for fingerprint or landmark calibration, frequent signaling between the mobile users and infrastructure, and rigorous application scenarios.

Based on the previous work on the location tracking in Wi-Fi environments (Ouyang et al., 2012, Ouyang et al., 2010, Zhou et al., 2013, Zhou et al., 2013), we can find that the Wi-Fi technology has the advantages of high accuracy, low cost, and widespread deployment. Moreover, the Wi-Fi modules can be integrated in most of current personal mobile devices, such as the laptops, cell phones, tablets, and personal digital assistants (PDAs). Therefore, after the world’s pioneering Wi-Fi location tracking system, RADAR, was invented by Microsoft Research in 2000 (Bahl & Padmanabhan, 2000), many universities and institutes began to develop a variety of prototype and commercial systems to track people’s locations (Castro et al., 2003, Garcia-Valverde et al., 2013, Swangmuang and Krishnamurthy, 2008, Youssef and Agrawala, 2008).

So far, there are generally four types of location tracking techniques by using the Wi-Fi networks: the time or time difference of arrival (TOA or TDOA) (Golden and Bateman, 2007, Schwalowsky et al., 2010), angle of arrival (AOA) (Wong et al., 2007, Wong et al., 2008), propagation model (Ahn and Yu, 2008, Emery and Denko, 2007), and received signal strength (RSS)-based fingerprinting (Bahl and Padmanabhan, 2000, Castro et al., 2003, Swangmuang and Krishnamurthy, 2008, Youssef and Agrawala, 2008). By TOA or TDOA, strict time synchronization of access points (APs) and mobile stations within a network is required (Golden and Bateman, 2007, Schwalowsky et al., 2010). However, multipath fading and shadowing in target environment make it difficult to obtain the exact arrival time of signal and propagation distance. In other words, the results of location tracking obtained by the combination of log-distance path loss model with triangulation algorithm cannot be accurate. For AOA technique, it is suggested that the smart antenna is used at the receiver to track locations by measuring the angles of arrival signal (Wong et al., 2007, Wong et al., 2008). In all, the TOA or TDOA and AOA techniques heavily rely on the sophisticated propagation model and extra hardware to achieve location tracking.

In comparison, the Wi-Fi RSS-based fingerprinting is preferred for location tracking because there is no requirement for any extra devices besides the widely deployed Wi-Fi networks in public. In general, there are two phases involved in the tracking process, the off-line phase and on-line phase (Fang and Lin, 2010, Zhou et al., 2012). In the off-line phase, the fingerprints from the RSS measurements at reference points (RPs) and the corresponding physical coordinates are recorded to construct a radio map. Then, in the on-line phase, the people’s locations are estimated by matching the pre-stored fingerprints with the new RSS data. The major drawback of RSS-based fingerprinting is the intensive labor cost for the fingerprint calibration.

To solve this problem, we propose a novel location tracking system called SCaNME which can adaptively construct an unlabeled mobility map and track people’s locations and paths without any calibration of fingerprints. The research presented in this study is an extension of our previous work (Zhou et al., 2013). Different from the conventional radio map, the unlabeled mobility map is represented by a graph of RSS clusters associated with non-zero transition probabilities. Meanwhile, by using the Allen’s logics on the temporal relations of time stamped RSS clusters, the people’s activities can be recorded from the mobility map. In this paper, we mainly focus on the activity recording for the people from the same organization. Our proposed SCaNME is beneficial to the future user-centric ubiquitous LBS which requires not only the knowledge of people’s real-time location information, but also the understanding of people’s routine activities. In the environments where the people’s movement patterns have been recorded, we can use SCaNME for loss finding, such as Find my phone at iphone. On the other hand, for healthcare applications, by tracking the movement of elderly people, the nursing center can monitor the activities of their care-receivers, or the families of the elder can track their locations anytime on-line in cloud. In all, the activity recording-based LBS could make our life easier and securer. Another application of SCaNME system is to describe the layout of anonymous environments adequately and adaptively. The off-line RSS training involved in conventional fingerprint-based location tracking systems requires significant time cost and periodic RSS management. To guarantee the efficiency of RSS training, a rough floor plan could be essential to the creation of off-line RSS database. However, in many anonymous environments, the floor plans are probably not available to the users for the security or privacy reasons. To solve this problem, the mobility map constructed by SCaNME system can be used to describe the target environmental layout in an effective manner. The SCaNME consists of the off-line map training phase and the on-line location tracking phase. The objectives of the off-line map training phase are to construct an unlabeled mobility map and record the people’s motion behaviors. During the on-line location tracking phase, the people’s locations and paths are tracked in a real-time manner.

The remaining of this paper is organized as follows. Section 2 summarizes some related work on Wi-Fi location tracking and describes the purpose of mobility map construction in SCaNME system. Section 3 explains the steps of tracking people’s locations by SCaNME system. In Section 4, we discuss the way to record the people’s activities from the unlabeled mobility map by Allen’s logics. Section 5 provides some numerical results collected from the experiments in a campus-wide Wi-Fi network. In Section 6, we conclude the paper and present our future work.

Section snippets

Related work

In the recent decade, the proliferation of ubiquitous LBS demand has fostered the research of Wi-Fi location tracking in both industry and academia. The fundamental model for the RSS-based location tracking was built by Microsoft Research’s RADAR system (Bahl & Padmanabhan, 2000). In RADAR system, the ideas of RSS-based fingerprinting and radio propagation model for the location tracking are introduced, which also become the two leading schemes to locate the people’s positions in Wi-Fi

Architecture of SCaNME location tracking

There are two phases involved in SCaNME system, the off-line mobility map construction phase and the on-line path reconstruction phase. An overview of our proposed SCaNME system is illustrated in Fig. 1. After the mobility map is constructed, we track the people’s positions by matching the newly collected RSS data into the pre-stored RSS samples. Details of each step in SCaNME system are described below.

Activity learning by Allen’s logics

In addition to the steps of location tracking in SCaNME system, the SCaNME also instantaneously learns the person’s activities from the previously reconstructed paths, in order to accurately and effectively locate the tracking LPs on the paths. To meet this goal, we use Allen’s logics as a new metric to investigate the person’s motion activities conveyed from the mobility map. Therefore, the probability of a LP being the tracking LP should also depend on the person’s prior knowledge of

Data collection

The efficiency and effectiveness of the location tracking by the SCaNME system is verified in one part of the HKUST campus (http://publish.ust.hk/univ/campu). The volunteers carrying the Samsung GT19100 Android phones collected 2017 Wi-Fi RSS measurements (of dimension 588) following their routine activities on Monday. They used the same wireless card vendor during both the off-line and on-line phases to reduce the ambiguity of displayed RSS values (Kaemarungsi, 2006, Mengual et al., 2010). In

Conclusion

Location tracking in large-scale environments is addressed as a passport to the all-weather and seamless LBSs in future. How to use widely-deployed Wi-Fi networks to perform the highly accurate and cost efficient location tracking remains to be an open issue. The conventional Wi-Fi RSS-based location fingerprinting involves a plethora of time and laboring cost for fingerprint recording. However, in SCaNME, we first use the sporadically recorded Wi-Fi RSS measurements to create a mobility map in

Acknowledgement

The authors wish to thank the reviewers and editors for their valuable comments and useful corrections. This work was done when Mu Zhou was a Post-doctoral Research Assistant at The Hong Kong University of Science and Technology (HKUST). This work was supported in part by the National Natural Science Foundation of China (61301126, 61304197), National Science and Technology Major Project of China (2011ZX03006003007, 2012ZX03006002003), Fundamental and Frontier Research Project of Chongqing (

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